1. Field of the Invention
The present invention relates to a differential input circuit, and particularly to a differential input circuit having CMOS circuits for high-speed signal transmission.
2. Description of the Related Art
Small-amplitude signal transmission has been widely used in CMOS circuits with an increase in a speed of signal transmission. The small-amplitude signal transmission is a technique which transmits signals so that the transmitted signals are not saturated, that is, switching transistors are not saturated. In the small-amplitude signal transmission, since transmitted signals are easily affected by a noise, differential signals are generally used.
In an interface system called LVDS (Low Voltage Differential Signals), small-amplitude signals having an amplitude of 100 mV at minimum are inputted and a logic level of the inputted signals must be determined. Additionally, the inputted signals have a wide voltage range ranging from 0 V to 2.4 V.
FIG.1 shows an example of a conventional differential input circuit. Transistors Tr11 and Tr12 constitute a current mirror circuit with their gates being connected to each other, and supply currents having the same amplitude to transistors Tr13 and Tr14. Precisely, the amplitudes of the currents supplied to the transistors Tr13 and Tr14 are proportional to widths of gates of the transistors Tr11 and Tr12, respectively. Thus, if the transistors Tr11 and Tr12 are designed to have the same physical dimension, currents having the same amplitude are supplied to the transistors Tr13 and Tr14.
An output voltage V.sub.out at an output terminal Out is changed when ON-state resistances between a drain and a source of the transistors Tr13 and Tr14 are changed in accordance with gate voltages of the transistors Tr13 and Tr14, which gate voltages correspond to input voltages of the circuit. When an input voltage V.sub.P at a P.sub.-in terminal connected to a gate of the transistor Tr13 is larger than an input voltage V.sub.N at an N.sub.-in terminal connected to a gate of the transistor Tr14, the output voltage V.sub.out is of a high level, and when V.sub.P is smaller than V.sub.N, the output voltage V.sub.out is of a low level. A transistor 15 which is connected to sources of the transistors Tr13 and Tr14 functions to define a total current flowing through the circuit. That is, the transistor 15 functions as a constant current source.
However, the above-mentioned conventional circuit can operate only when a common voltage (a center value of differential input signals) is within a limited range. FIG. 2 is a diagram for illustrating the common voltage. The common voltage is defined as a center value V.sub.ic of the positive input voltage V.sub.P and the negative input voltage V.sub.N of a differential input circuit U.
For example, when both the input voltages V.sub.P and V.sub.N are so high that the transistor Tr15 is saturated, both the transistors Tr13, Tr14 are turned on. In this case, the output voltage V.sub.out is fixed to be of the low level. On the other hand, if both the input voltages V.sub.P and V.sub.N are lower than a threshold voltage of the transistors Tr13 and Tr14, both the transistors Tr13 and Tr14 are turned off. In this case, the output voltage V.sub.out is indefinite.